Compositions and methods for treating subterranean formations

ABSTRACT

Methods for treating subterranean wells in which the producing formation is a carbonate comprise preparing a treatment fluid comprising either: (a) an aqueous solution of a multivalent-cation reacting polymer; (b) a solution comprising a fatty acid and an ethanolamine; (c) an aqueous acidic solution of one or more compounds whose calcium salts are insoluble; or (d) an aqueous solution comprising urea or alkane derivatives thereof or both and polyvinylpyrrolidone (PVP). The treatment fluid is placed in the well such that the solution contacts the carbonate formation at a pressure lower than the fracturing pressure. The treatment fluid is allowed to react with the carbonate formation, thereby depositing a film onto the formation surface or part of the formation surface. Then an acid solution is placed in the well such that the acid contacts the carbonate formation at a pressure lower than the fracturing pressure.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

This disclosure relates to compositions and methods for treatingsubterranean formations, stimulation treatments in particular.

Stimulation treatments in subterranean wells may be performed to improvethe rate at which hydrocarbons (or other valuable resources) areproduced, maximize the total amount of hydrocarbons that are produced orboth. Such treatments may be divided into two principal categories.Matrix stimulation treatments are those during which fluids areintroduced or pumped into the well at rates and pressures that do notfracture the rock surrounding the wellbore. Fracturing treatments arethose during which fluids are introduced or pumped into the well atrates and pressures that cause the rock surrounding the wellbore tofracture. The fractures may then propagate away from the wellbore andincrease the surface area of rock available to the wellbore by orders ofmagnitude.

Fluids employed during stimulation treatments may contain acids that maydissolve and increase the permeability of producing formations.Hydrochloric acid may be used to stimulate carbonate formations, whilehydrofluoric acid may be used to treat sandstone formations. Otherorganic acids such as formic acid and acetic acid may also haveapplication.

Carbonates are sedimentary rocks deposited in a marine environment andare largely of biological origin. The nature of local depositioncontributes to their extensive heterogeneity across all scales—frompores, to grains and to textures. Carbonate rocks may exhibit a complextexture and pore network resulting from their depositional history andsubsequent diagenesis.

Matrix acidizing and fracture acidizing are conventionally used torelease hydrocarbons in such formations, but the stimulation may beimpeded by a lack of heterogeneity of carbonate surfaces. Surfaceheterogeneity favors the creation of a more complex network ofconductive channels (or “wormholes”) as the acid penetrates theformation, and may maximize production efficiency.

Hydrochloric acid (HCl) at a 15 wt % concentration, accompanied by asuitable level of corrosion inhibitor to reduce corrosion to completionand metallic tools, may be used in acid stimulation operations. Theprincipal chemical reaction is shown in Eq. 1.

2HCl+CaCO₃→CaCl₂+CO₂+H₂O   (Eq. 1)

A number of difficulties may arise due to the high sensitivity of theentire carbonate formation, which by nature may be highly uneven inpermeability distribution, to a strong mineral acid system. Notably,etching of the formation by the acid may be confined to locations thatare already permeable. Highly permeable areas attract more acid flow dueto decreased diffusion limitations, which in turn creates largerwormholes that consume more acid, leaving low-permeability areasuntouched. Therefore, the network of wormholes may not be optimal andacid may be unnecessarily wasted.

Conventional acidizing fluids, such as hydrochloric acid or a mixture ofhydrofluoric and hydrochloric acids, have high acid strength and mayreact quickly with fines and scale nearest the wellbore, and may have atendency to corrode tubing, casing and downhole equipment, such asgravel pack screens and downhole pumps, especially at elevatedtemperatures. In addition, above 200° F. (93 ° C.), straight HCl is notrecommended in some cases because of its destructive effect on the rockmatrix. Due to the type of metallurgy, long acid contact times and highacid sensitivity of the formations, scale removal with hydrochloric acidand hydrochloric acid mixtures has been largely unsuccessful.

SUMMARY

In an aspect, embodiments relate to treatment fluid systems. The systemscomprise a first fluid comprising either:

an aqueous solution of a multivalent-cation reacting polymer;a solution comprising a fatty acid and an ethanolamine;an aqueous acidic solution of one or more compounds whose calcium saltsare insoluble; oran aqueous solution comprising urea or alkane derivatives thereof orboth, and polyvinylpyrrolidone (PVP). The systems further comprise asecond fluid that comprises an acid.

In a further aspect, embodiments relate to methods for treating asubterranean well having a borehole and a carbonate formation with afracturing pressure. A treatment fluid is prepared that compriseseither:

an aqueous solution of a multivalent-cation reacting polymer;a solution comprising a fatty acid and an ethanolamine;an aqueous acidic solution of one or more compounds whose calcium saltsare insoluble; oran aqueous solution comprising urea or alkane derivatives thereof orboth, and polyvinylpyrrolidone (PVP). The treatment fluid is placed intothe well such that the solution contacts the carbonate formation at apressure lower than the fracturing pressure. The treatment fluid isallowed to react with the carbonate formation, thereby forming a filmonto the formation surface or parts of the formation surface. An acidsolution is then placed in the well such that the acid contacts thecarbonate formation at a pressure lower than the fracturing pressure.

In yet a further aspect, embodiments relate to methods for stimulating asubterranean well having a borehole and a carbonate formation having afracturing pressure. A treatment fluid is prepared that compriseseither:

an aqueous solution of a multivalent-cation reacting polymer;a solution comprising a fatty acid and an ethanolamine;an aqueous acidic solution of one or more compounds whose calcium saltsare insoluble; oran aqueous solution comprising urea or alkane derivatives thereof orboth, and polyvinylpyrrolidone (PVP). A first acid solution is placedinto the well such that the solution contacts the carbonate formation ata pressure lower than the fracturing pressure. The treatment fluid isplaced into the well such that the treatment fluid contacts thecarbonate formation at a pressure lower than the fracturing pressure.The treatment fluid is allowed to react with the carbonate formation,thereby forming a film onto the formation surface or parts of theformation surface. A second acid solution is then placed in the wellsuch that the acid contacts the carbonate formation at a pressure lowerthan the fracturing pressure. Placement of the treatment fluid and theacid may be repeated until a desired amount of stimulation is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of alginic acid building blocks, M and G,and their distribution.

FIG. 2 is a drawing depicting the passivation of calcite surfaces bycrosslinked algninic acid.

FIG. 3 presents plots showing the shear stress and viscosity of variousalginate/water solutions as a function of shear rate.

FIG. 4 presents plots showing the shear stress and viscosity of aqueoussolutions of alginates from two different suppliers.

FIG. 5 is a plot showing the volume of CO₂ evolved when Indianalimestone disks (untreated and treated with alginate) are contacted byHCl.

FIG. 6 is a plot showing the rate at which CO₂ forms when Indianalimestone disks (untreated and treated with alginate) are contacted byHCl.

FIG. 7 is a series of plots showing passivation factors as a function oftime for calcite surfaces treated with various alginate compositions.

FIG. 8 is a plot showing the effect of rotational speed on thereactivity of limestone cores during rotating disk experiments.

FIG. 9 is a plot showing the acid consumption rate as a function ofrotational speed during rotating disk experiments.

FIG. 10 is a plot showing the mass of calcite lost during a two-stagereaction test for various salt additives at a concentration of 5 wt %,normalized by the mass lost during a 10 wt % HCl pure acid treatment.

FIG. 11 is a plot showing the effect of Na₂SO₄ concentration on the massof air dried calcite specimens treated with 15 wt % HCl.

FIG. 12 is a plot showing the effect of Na₂SO₄ concentration on the massof oven dried calcite specimens treated with 15 wt % HCl.

FIG. 13 is a plot showing the effect of Na₂SO₄ concentration on thethickness of oven dried calcite specimens treated with 15 wt % HCl.

FIG. 14 is a plot showing the effect of Na₂SO₄ concentration on theextent of calcite specimen reaction versus time.

FIG. 15 is a plot that compares the reaction rates over time for variouslevels of Na₂SO₄ addition, normalized by the square of the quantity ofHCl remaining.

FIG. 16 is a plot showing the mass loss from calcite specimens treatedwith HCl alone and HCl+5% Na₂SO₄.

FIG. 17 is a series of plots that compare the generation of Ca²⁺ ion inthe presence of pure 15 wt % HCl versus 15 wt % HCl+5 wt % Na₂SO₄.

DETAILED DESCRIPTION

The present disclosure will be described in terms of treatment ofvertical wells, but is equally applicable to wells of any orientation.The disclosure will be described for hydrocarbon-production wells, butit is to be understood that the disclosed methods can be used for wellsfor the production of other fluids, such as water or carbon dioxide, or,for example, for injection or storage wells. It should also beunderstood that throughout this specification, when a concentration oramount range is described as being useful, or suitable, or the like, itis intended that any concentration or amount within the range, includingthe end points, is to be considered as having been stated. Furthermore,each numerical value should be read once as modified by the term “about”(unless already expressly so modified) and then read again as not to beso modified unless otherwise stated in context. For example, “a range offrom 1 to 10” is to be read as indicating each possible number along thecontinuum between about 1 and about 10. In other words, when a certainrange is expressed, even if just a few specific data points areexplicitly identified or referred to within the range, or even when nodata points are referred to within the range, it is to be understoodthat the Applicants appreciate and understand that any and all datapoints within the range are to be considered to have been specified, andthat the Applicants have possession of the entire range and all pointswithin the range.

In this disclosure, the tubular body may be any string of tubulars thatmay be run into the wellbore and at least partially cemented in place.Examples include casing, liner, solid expandable tubular, productiontubing and drill pipe.

The present disclosure reveals compositions and procedures forselectively passivating carbonate surfaces. Such carbonate surfaces mayinclude calcite, aragonite, vaterite and dolomite.

Compositions may comprise “multivalent cation reacting polymer” as usedherein encompasses polymer that react in presence of multivalent cationsuch as calcium. Non limiting examples are caraggenan, pectin,carboxylate containing polymer, such as carboxymethylcellulose (CMC),carboxymethyl hydroxypropyl guar (CMHPG), carboxymethyl hydroxyethyylguar (CMHEG), xanthan, diutan, carboxymethylhydroxylcellulose (CMHEC),carboxymethyl starch, partially hydrolyzed polyacrylamide (PHPA),acrylic acid and its derivatives. In embodiments the multivalent-cationreacting polymer. In embodiments, the compositions may comprise alginicacid (ALG), a biopolymer readily available from brown algae. Alginatesare heterogeneous in their composition, which varies with differentalgal sources and by which part of the seaweed the alginate washarvested from. The polysaccharide has two building blocks: D-mannuronic(M) and L-guluronic acid (G), a schematic drawing of which is shown inFIG. 1. The M-to-G ratio, as well as the block sequencing, may affectthe physical properties of the polymer. ALG with a counterion to thecarboxylate group forms a viscous solution in an aqueous solvent.However, introduction of multivalent cations may result in crosslinkingof the linear polymer to form thick, translucent gels. Such a protocolinvolves using ALG to protect calcite surfaces for at least a two-foldeffect: (1) restricting acid access to the carbonate surface; and (2)causing selective etching. FIG. 2 shows a schematic depiction of howadhesion of ALG to calcite surface sites may cause passivation to partsof the surface while allowing HCl to access other, uncovered parts. Thepresence of calcium cation, produced live by acid etching on calcite,forms a more viscous crosslinked gel with ALG.

The M-to-G ratio may be between 0.15 and 2.00. The M-to-G blocksequencing may vary between MMMMMMM, GGGGGGG and MGMGMG. Furthermore,the molecular weight of the alginates may be between 5,000 and 800,000.

Further compositions may comprise a fatty acid and an ethanolamine.Fatty acid molecules have the basic chemical formula CH₃(CH₂)_(n)COOH,and possess a hydrophilic acidic end and a hydrophobic alkyl end. Whenthey contact a calcium carbonate surface, the acidic end may becomeaffixed to the surface. Without wishing to held to any particulartheory, the interaction may result from an electrostatic attractionbetween Ca²⁺ and RCOO⁻, forming a film comprising the calcium salt ofthe fatty acid. Suitable fatty acids include, for example, stearic acid,palmitic acid, behenic acid, lauric acid, erucic acid and oleic acid.The ethanolamine may be monoethanolamine, diethanolamine, ortriethanolamine or combinations thereof

Yet a further composition may comprise compounds whose calcium salts areinsoluble. Without wishing to be held to any particular theory, whenthese compounds contact a calcium carbonate surface, they react to forma calcium salt that forms a protective coating that diverts acid to lesspermeable regions. Such compounds may include sodium sulfate, sodiumphosphate dibasic, sulfamic acid, sodium-4-hydroxybenzenesulfonate,oxalic acid, tetrapotassium pyrophosphate, potassium periodate orpotassium phosphate monobasic or combinations thereof. The concentrationof the compounds whose calcium salts are insoluble may be between 0.05%and 30.0%, or between 0.10% to 15.0%, or between 0.10% to 7.50%, orbetween 0.10% to 2.00% by weight.

Yet a further composition may comprise an aqueous solution comprisingurea, alkane derivatives thereof or both, and polyvinylpyrrolidone(PVP). The concentration of polyvinylpyrrolidone (PVP) may be in therange of from 0.5 wt % to 7.5 wt %, or 0.75 wt % to 5.0 wt %, or 1.0 wt% to 2.5 wt %.

The term “acid” as used herein encompasses mineral and organic acids.Examples of mineral acids are hydrochloric acid (HCl), nitric acid,phosphoric acid, sulfuric acid, boric acid, hydrofluoric acid,hydrobromic acid, perchloric acid, or derivatives, and mixtures thereof.The acid may be transported to a wellsite, the acid having aconcentration between 28 wt % and 45.7 wt %. The acid may then bediluted to a treatment concentration before providing the oilfieldtreatment fluid to the high-pressure pump. When HCl is used, the acidsolution may contain HCl at concentrations between 7.5 wt % and 28 wt %.

Examples of organic acids are formic acid, citric acid, acetic acid,glycolic acid, dicarboxylic acids (including oxalic acid, malonic acid,succinic acid, glutaric acid, adipic acid, pimelic acid and the like),tricarboxylic acids, aminocarboxylic acids, chloroacetic acid,hydroxyacetic acid and mixtures thereof may also have application.Organic acid derivatives may also be considered. For example, aderivative of acid may be methanesulfonic acid or its alkane, benzene ornaphthalene derivatives. When organic acids are used, theirconcentration may range between 5.0 wt % and 30 wt %.

In an aspect, embodiments relate to treatment fluid systems. The systemscomprise a first fluid comprising either:

an aqueous solution of a multivalent-cation reacting polymer;a solution comprising a fatty acid and an ethanolamine;an aqueous acidic solution of one or more compounds whose calcium saltsare insoluble; oran aqueous solution comprising urea or alkane derivatives thereof orboth, and polyvinylpyrrolidone (PVP). The systems further comprise asecond fluid that comprises an acid. Such systems may be used to modifythe nature of carbonate mineral surfaces.

In a further aspect, embodiments relate to methods for treating asubterranean well having a borehole and a carbonate formation with afracturing pressure. A treatment fluid is prepared that compriseseither:

an aqueous solution of a multivalent-cation reacting polymer;a solution comprising a fatty acid and an ethanolamine;an aqueous acidic solution of one or more compounds whose calcium saltsare insoluble; oran aqueous solution comprising urea or alkane derivatives thereof orboth, and polyvinylpyrrolidone (PVP). The treatment fluid is placed intothe well such that the solution contacts the carbonate formation at apressure lower than the fracturing pressure. The treatment fluid isallowed to react with the carbonate formation, thereby forming a filmonto the formation surface. An acid solution is then placed in the wellsuch that the acid contacts the carbonate formation at a pressure lowerthan the fracturing pressure.

The treatment fluid may be allowed to contact the carbonate surface forperiods up to one hour.

In yet a further aspect, embodiments relate to methods for stimulating asubterranean well having a borehole and a carbonate formation having afracturing pressure. A treatment fluid is prepared that compriseseither:

an aqueous solution of a multivalent-cation reacting polymer;a solution comprising a fatty acid and an ethanolamine;an aqueous acidic solution of one or more compounds whose calcium saltsare insoluble; oran aqueous solution comprising urea or alkane derivatives thereof orboth, and polyvinylpyrrolidone (PVP). A first acid solution is placedinto the well such that the solution contacts the carbonate formation ata pressure lower than the fracturing pressure. The treatment fluid isplaced into the well such that the treatment fluid contacts thecarbonate formation at a pressure lower than the fracturing pressure.The treatment fluid is allowed to react with the carbonate formation,thereby forming a film onto the formation surface. A second acidsolution is then placed in the well such that the acid contacts thecarbonate formation at a pressure lower than the fracturing pressure.Placement of the treatment fluid and the acid may be repeated until adesired amount of stimulation is achieved.

The treatment fluid may be allowed to contact the carbonate surface forperiods up to one hour.

For each aspect, the multivalent-cation reacting polymer may be presentat a concentration between 0.1% and 3.0% by weight of the solution.

For each aspect, the ethanolamine may comprise monoethanolamine,diethanolamine, or triethanolamine or combinations thereof.

For each aspect, the fatty acid may have the chemical formulaCH₃(CH₂)_(n)COOH, wherein n is between 10 and 24 inclusive, and thefatty acid is present at a concentration between 1.0% and 2.5% byweight. The fatty acids may comprise stearic acid, palmitic acid,behenic acid, lauric acid, erucic acid or oleic acid or combinationsthereof

For each aspect, the compounds whose calcium salts are insoluble maycomprise sodium sulfate, sodium phosphate dibasic, sulfamic acid,sodium-4-hydroxybenzenesulfonate, oxalic acid, tetrapotassiumpyrophosphate, potassium periodate or potassium phosphate monobasic orcombinations thereof, wherein the compounds are present at aconcentration between 1.0% and 10% by weight.

For each aspect, the treatment fluid may further comprise sulfonic acidor derivatives thereof or both. Fluid formulations containingmethanesulfonic acid, or one of its alkane or benzene or naphthalenederivatives, or mixtures thereof, may have improved thermal stability.These materials may be present at concentrations between 0.1% and 1.0%by weight.

When the acid solution is made of hydrochloric acid, the solution maycontain HCl at concentrations between 7.5 wt % and 28 wt %. Thehydrochloric acid solution may further comprise urea or one or more ureaderivatives, or both, at concentrations between 7.0 wt % and 46 wt %.

EXAMPLES

The following examples are provided to more fully illustrate thedisclosure. These examples are not intended to limit the scope of thedisclosure in any way.

Example 1

Solutions of sodium alginate (Na-ALG) were prepared by blending sodiumalginate (available from Acros—Product No. MFCD00081310) with deionizedwater to obtain a desired weight concentration (0.1%-3.0%). A Waringblender was used. Blending powdered ALG with water produced essentiallyhomogeneous mixtures within 30 min. Attention was paid to the sides ofthe blender cup, as the ALG powder tended to stick to the sides of thevessel and avoid dissolution. Using a stirring bar, sonication bath, orsimply shaking by hand was not effective.

Addition of acid (e.g. HCl) to solutions of sodium alginate caused aphase change of the alginic acid, owing to protonation of the carboxylicgroup. Tests were conducted by adding concentrated HCl (37%) to ALGsolutions such that the final HCl concentration was 15%. While blending,addition of HCl converted the homogeneous polymer fluids into clumpy,translucent gels.

Addition of calcium (CaCl₂) to the ALG solutions lead to immediatecrosslinking of the linear polymer to form solid and translucent gels.Different addition methods resulted in different gel structures.Introduction of CaCl₂ dropwise caused “beads” of cross-linked ALG toform; introduction of a CaCl₂ stream under low rotation caused “noodles”of cross-linked ALG to form; introduction of CaCl₂ while under highrotation caused “clumps” of cross-linked ALG to form. In all cases, highALG-to-Ca weight ratios were employed (ALG:Ca: up to 10³:1). Continuousaddition of 1% CaCl₂ while blending caused the ALG to bind all availableCa²⁺ ions to form a solid gel with no liquid fraction. The mixturereached a saturation point when no additional Ca²⁺ could be adsorbed,causing a liquid fraction to appear when more aqueous CaCl₂ was added.For effective treatment of carbonate surfaces, it may be optimal tosupply ALG and Ca²⁺.

Example 2

Rheological measurements of alginate solutions were conducted todetermine how the viscosity changed by varying the ALG concentrations aswell as the effect of different additives. FIG. 3 shows the shear stressand viscosity as a function of shear rate for solutions containing from0.5 wt % to 3.0 wt % ALG in water. As predicted by most polymersolutions, the shear stress and viscosity increased with increasing ALGconcentration. Rheological tests were also conducted on ALG solutionsmade from alginate obtained from different suppliers. FIG. 4 comparesthe shear stress and viscosity of alginate solutions from Acros andSigma-Aldrich. A decrease in viscosity was observed in the Sigma-Aldrichsamples. The physical properties were highly dependent on the algaesource as well as the molecular weight and the M and G block sequencing.The M to G ratio is dependent on the algae species and what part of theplant the alginate was harvested from (stem versus leaf). In FIG. 4,data from the Acros and Sigma-Aldrich samples are depicted as circlesand squares, respectively. Open squares and circles show data from 1 wt% alginate solutions, and closed squares and circles show data from 2 wt% alginate solutions.

Example 3

Indiana limestone disks of 1-in. (2.54-cm) diameter and 0.25-in.(0.64-cm) thickness were treated by various ALG-containing solutionsdescribed below.ALG: Disk submersion in 1 wt % ALG solution, vacuum chamber for 2 h.ALG/CaCl₂: Disk submersion in 1 wt % ALG solution, vacuum chamber for 2h, subsequent disk submersion in 1 wt % CaCl₂, vacuum chamber for 2 h.CaCl₂/ALG: Disk submersion in 1 wt % CaCl₂ solution, vacuum chamber for2 h, subsequent disk submersion in 1 wt % ALG, vacuum chamber for 2 h.ALG-Na₂SO₄: Disk submersion in 1 wt % ALG/1 wt % Na₂SO₄ solution, vacuumchamber for 2 h.ALG-LUDOX™: Disk submersion in 1 wt % ALG/1 wt % LUDOX™HSA solution(colloidal silica available from WR Grace & Co.), vacuum chamber for 2h.ALG-Na₂SiO₃: Disk submersion in 1 wt % ALG/1 wt % Na₂SiO₃ solution,vacuum chamber for 2 h.

The solutions were placed in a vacuum chamber to facilitate solutionsaturation into the pore structures before being immersed in 15 wt %HCl. These treatments often left a gel layer of crosslinked ALGsurrounding the disk. Such a layer was susceptible to physical damage;therefore, care was taken to limit testing to samples with structurallysound gel layers and compare those results across different treatmentmethods. The treated cores were then exposed to 30 g of 15 wt % HCl for10 min at ambient temperature, and the limestone weight loss (Eq. 2) wasrecorded, allowing a comparison among the different treatment methods.Table 1 shows the percent limestone mass loss as a function of thetreatment method.

$\begin{matrix}{{\% \mspace{14mu} {Loss}} = {\frac{{Mass}_{initial} - {Mass}_{final}}{{Mass}_{initial}} \times 100}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

The passivation factor given in Table 1 is defined as the reciprocal ofthe mass loss ratio with a given ALG-containing solution as compared toALG-free solution.

TABLE 1 Percent mass losses and passivation factors of Indiana limestonepre-treated with ALG-containing solutions after HCl reaction. FluidFormation Mass Loss Passivation Factor ALG 7.1% 14.2 ALG/CaCl₂ 25.1% 4.0CaCl₂/ALG 11.5% 8.7 ALG-Na₂SO₄ 41.9% 2.4 ALG-LUDOX ™ 5.7% 17.5ALG-Na₂SiO₃ 91.1% 1.1 No Treatment 100.0% 1.0

Each ALG-containing solution demonstrated resistance to limestone massloss arising from strong mineral acid etching. While the effect withALG-silicate nanoparticles was marginal, the passivation factor was ashigh as 17.5 in the case of an ALG-LUDOX™ mixture.

Example 4

In order to measure transient reaction rates, a simple eudiometerapparatus was constructed to measure the volume of CO₂ evolved duringlimestone reactions with HCl. The Indiana limestone disks were treatedby an ALG solution before being immersed in 30 g of 15% HCl for 10 min.The treatment solutions are listed below.

1% ALG: Disk submersion in 1% HCl for 5 seconds, disk submersion in 1%ALG solution, vacuum chamber for 2 hours.1% ALG/1% PVP: Disk submersion in 1% HCl for 5 seconds, disk submersionin 1% ALG/1% polyvinylpyrrolidone solution, vacuum chamber for 2 hours.1% ALG/5% Urea: Disk submersion in 1% HCl for 5 seconds, disk submersionin 1% ALG/5% urea solution, vacuum chamber for 2 hours.1% ALG/5% EU: Disk submersion in 1% HCl for 5 seconds, disk submersionin 1% ALG/5% ethylene urea solution, vacuum chamber for 2 hours.1% ALG/5% DMU: Disk submersion in 1% HCl for 5 seconds, disk submersionin 1% ALG/5% N,N′-dimethyl urea solution, vacuum chamber for 2 hours.1% ALG/5% BHMU: Disk submersion in 1% HCl for 5 seconds, disk submersionin 1% ALG/5% N,N′-bis(hydroxyl methyl) urea solution, vacuum chamber for2 hours.

The activated Ca²⁺ ions as a result of HCl etching were immediatelyavailable for ALG cross-linking. The evolution of CO₂ was measured atdifferent times as the gas replaced water in the eudiometer tube or agraduated cylinder. No corrections were made for H₂O vapor pressure,barometric pressure, temperature, or CO₂ solubility in H₂O. CO₂ volumeswere measured every 10-15 s up to 120 s of reaction, then measured every30 s thereafter, until 600 s had elapsed. After 600 s, the limestonedisks were removed from the HCl solution and rinsed with H₂O to quenchthe reaction. FIG. 5 shows the evolution of CO₂ as a function ofreaction time for various treated cores. The generated CO₂ volumesvaried, showing a strong passivation effect of ALG on calcite.

By converting the volume of evolved CO₂ into moles, using the density ofa gas at 20° C. (24 L/mol), it was possible to calculate the rates ofCO₂ evolution in moles as a function of time (Eq. 3). FIG. 6 presentsthe rate of CO₂ formation as a function of reaction time for untreatedlimestone, as well as two ALG-treated limestone disks. Initial rates ofCO₂ formation on the untreated disk exceeded 3×10⁻⁴ mol s⁻¹, thendecreased with time to approximately 3×10⁻⁵ mol s⁻¹. The decrease information rate was caused by decreasing acid concentration anddecreasing limestone surface availability. The ALG and ALG/Urea treateddisks had much lower rates, approximately 2 and 4−10⁻⁶ mol s⁻¹,respectively. These rates were largely constant throughout the entirereaction time. Acid consumption was correspondingly lower duringreactions with treated disks, as shown in Table 2.

$\begin{matrix}{{Rate} = {\frac{\Delta \; {Mole}}{\Delta \; {Time}} = \frac{{Mole}_{x + {\Delta \; x}} - {Mole}_{x}}{{Time}_{x + {\Delta \; x}} - {Time}_{x}}}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

TABLE 2 Final solution concentrations of HCl, disk weight losses and λsfor Indiana limestone disks treated with various alginate formulations.Treatment Final HCl (wt %) Weight Loss (%) λ₁₂₀₊ Untreated 3.9 65.3 1 1%ALG 13.7 2.8 28 1% ALG/1% PVP 13.7 4.5 20 1% ALG/5% Urea 13.2 5.1 12 1%ALG/5% EU 13.8 4.2 44 1% ALG/5% DMU 13.8 3.4 32 1% ALG/5% BHMU 13.8 4.116

A passivation factor can be calculated which allows direct comparison ofpassivation enhancement between different treatment methods. Thepassivation factor, λ, is defined as the ratio of the rate of CO₂formation at time x for an untreated disk and the rate at time x for atreated disk, as shown in Eq. 4. FIG. 7 shows λ as a function ofreaction time for various treatment methods, which are described below.λ was initially very high as the diffusion of acid through the gel layerwas slow, causing a low rate of CO₂ formation. At longer reaction times,λ was fairly constant with time as CO₂ formation for the treated anduntreated disks trends towards steady state. An average λ was calculatedfrom all the λ values after 120 s of reaction. λ values before 120 minwere high and artificially increased the average values. Average λvalues allowed for a quick and simple comparison of passivationenhancement among varying treatment methods, as given in Table 2.

$\begin{matrix}{\lambda = \frac{{Rate}_{t = x}^{Untreated}}{{Rate}_{t = x}^{Treated}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

Example 5

Rotating disk experiments (RDE) were conducted to obtain kinetic rateparameters for acid consumption. RDE is a traditional method used todetermine rate parameters, both in the mass transfer and reaction ratelimited regimes. Disks were pretreated in 1% HCl prior to adhesion tothe RDE spindle with epoxy. The core and spindle were then submersed in1% ALG for 2 h under vacuum. The ALG film was visible at the top surfaceof the disk. Three experiments were conducted at 60, 500 and 1000 rpmeach at 150° F. (65.6° C.) with 15 wt % HCl as the reaction fluid. Fluidaliquots were taken every minute for five minutes. The fluid was thentested for Ca content via inductively coupled plasma (ICP) analysis.

The gel remained intact on the surface of the disk, while etching of thedisk edges occurred with increasing shear rate as the rotational speedincreased. The greater amount of etching at higher rpm was also evidentas the Ca concentration increased both with reaction time and rpm (FIG.8). The acid consumption rate was calculated from the amount of Caproduced. As shown in FIG. 9, this rate increased with the square rootof the rotational velocity, indicating the experiment is within the masstransfer limited regime. For comparison, the consumption rate of anuntreated core at the same reaction conditions was overlayed with the 1%ALG treated experiments. A two orders of magnitude decrease in the acidconsumption rate was observed, that was evident by observing the“leveling off” of the consumption rate for rotational velocities afterapproximately 25 s^(−1/2). The effect of the ALG-coating, resulted intotal surface area reduction, outweighed the rate of temperatureincrease.

Example 6

Stearic acid (SA), like other fatty acids, possesses both a hydrophilicacid end and a hydrophobic alkyl end. Theoretically, the acidic end willaffix to the calcium carbonate surface (potentially by the electrostaticattraction between Ca²⁺ and RCOO⁻) and form a calcium stearate (CaSt)film that is water insoluble. Other fatty acids may work in an identicalmanner, thus stearic acid, palmitic acid, behenic acid, lauric acid, andoleic acid were tested.

A 1 wt % SA/DEA (diethanolamine) solution was applied to one-half of thesurface of a rectangular Indiana limestone sample using an ordinarycotton swab. A sufficient quantity was used to thoroughly coat thesurface, but not so much as to allow fluid run-off when the core wastilted. Upon submersion in 1% HCl, a difference in reaction rate wasobserved between the treated and untreated halves. In fact, the treatedface had no visible bubbles of CO₂ evolution. After a period of up to 15min, the treated surface turned white; an opaque film of stearic acidadhered to the surface as the DEA solvent washed off. This film was notcalcium stearate, but pure stearic acid. Due to the higher solubility ofSA in DEA over H₂O, the solute is driven to the surface as the outer DEAlayers diffuse into aqueous solution. As the reaction continued, CO₂bubbles agitated the edges of the film sufficiently to lift off inpieces.

In addition, other analogs with similar alkyl chains were examined. Inorder to make a direct comparison between these compounds, standardsolutions were prepared in DEA at concentrations of 1 wt % and 2.5 wt %.Protection time was measured visually as the time required for a treatedcore to react at the same rate as an untreated core (that is, the timefor a portion of the treated surface to be washed off/removed whensubmerged in acid). Each core was a rectangular Indiana Limestonespecimen, prepared by applying a small quantity of solution with acotton swab. Pure solvent (DEA) was used as a reference.

TABLE 3 Summary of fatty acid/DEA solution interactions with HCl. Thefirst column represents protection time in seconds, while the second isthe relative passivation factor compared to pure DEA. 1% HCl/H₂O 5%HCl/H₂O Pure DEA 20 1.0 20 1.0  1% Stearic (18C) 720 36.0 20 1.0Palmitic (16C) 480 24.0 120 6.0 Behenic (22C) 720 36.0 20 1.0 Lauric(12C) 180 9.0 20 1.0 Oleic (18C) 240 12.0 90 4.5 2.5% Stearic (18C) 125062.5 240 12.0 Palmitic (16C) 945 47.3 40 2.0 Behenic (22C) 1200 60.0 402.0 Lauric (12C) 410 20.5 60 3.0 Oleic (18C) 525 26.3 60 3.0

Example 7

An Indiana limestone core was placed on a hot plate at approximately200° F. (93° C.), and treated by applying 5 wt % HCl droplets on thesurface that led to CO₂ gas generation and an excess of calcium cationsleft on the surface as the water solvent and reaction product penetratedinto the pore structure. Then, a 1 wt % aqueous sodium stearate solutionwas applied to the reacted surface. Upon drying, droplets of 15 wt % HClwere applied to the modified surface. The droplets remained largelydome-shaped due to the hydrophobic nature of the calcium stearatemonolayer. This process lasted for up to 120 s, after which the acideventually found its way through existing pinholes in the film andreacted with the limestone. This time period was sufficient to generateconsiderable heterogeneity on the carbonate surface.

Example 8

In a two-stage reaction, a rectangular limestone core was submerged in 5wt % HCl/10 wt % Na₂SO₄ for 3 minutes, and then submerged in pure 5 wt %HCl for 5 min. After the first treatment, there was a uniform white maskcovering the surface where the main acid etching reaction took place.After contacting 5 wt % HCl for a period of 5 min, small pockets beganto develop and the HCl started to penetrate through the mask reactingwith limestone underneath. Due to the isotropic nature of the reactionbetween HCl and CaCO₃, the proton may have functioned in a mannersimilar to a molecular drill into the core, accompanied by undercuttinginto areas underneath the mask, the integrity of which was eventuallycompromised.

In addition, a number of other compounds were screened. Chosen for theirpotential ability to form insoluble calcium salts, these included:sodium sulfate, sodium phosphate dibasic, sulfamic acid, sodium4-hydroxybenzenesulfonate, oxalic acid dehydrate, tetrapotassiumpyrophosphate, potassium periodate, potassium phosphate monobasic,Arsenazo III and phenol red. While some of these tests were limited todropping a dilute solution of salt on the surface and then reacting withacid, others were done more thoroughly. For those done more thoroughly,the test consisted of a two-stage treatment. First, cores were submergedin a 10 wt % HCl-5 wt % Na₂SO₄ solution for 2 min, then transferred to a15 wt % HCl bath for 2 additional min. Cores were weighed beforereaction, and after oven-drying. The concept is that the core develops aprotective coating during the first stage, and then benefits from somelevel of protection during the second stage. The normalized mass lossesfor limestone specimens coated with various compounds are shown in FIG.10.

Sodium sulfate appeared to be the most effective masking material. Therewas moderate fluctuation for other compounds, but this may have been dueto the inherent acidity/basicity of those compounds. Irrespective of theactual mechanism that made sulfate function better than other salts, itwas a promising candidate for surface masking and was chosen for furthertests.

Fluid formulations containing methanesulfonic acid, or one of its alkaneor benzene or naphthalene derivatives, or mixtures thereof, may haveimproved thermal stability. These materials may be present atconcentrations between 0.1% and 1.0% by weight.

Example 9

The reacting core surface profile was evaluated as the CaSO4 maskdeveloped over time. Cores were photographed during various stages ofthe treatment, then dried and measured for both mass and width. Testswere performed by submerging Indiana limestone disks (1-in [2.54-cm) D,⅛-in. [0.32-cm] H) in 30 mL of solution (1 wt %, 5 wt %, 10 wt %, and 15wt % Na₂SO₄ in 15 wt % HCl) with brief pauses for photographs at 0, 15,30, 45, 60, 90, 120, 210, 300, 450, and 600 s. A second round of testswas performed in 0 wt %, 0.5 wt %, 1 wt %, and 2 wt % Na₂SO₄ in 15 wt %HCl with photographs at 0, 30, 60, 120, 300, and 600 s.

Cores were held vertically upon removal from the treatment fluids toallow excess liquid to run off the surface. Despite this, some acidremained and reacted on the surface during the pauses, thus creatingsome experimental uncertainty.

After the reaction, the cores were air-dried for 2 h at ambienttemperature before weighing. The cores were weighed once more afterdrying overnight at 225° F. (107° C.). Although the results were notadjusted for initial masses, it is notable that the vast majority ofcores had initial masses between a narrow window of 6.9 and 7.1 grams.The total reaction time was 10 min for each core. The resulting masschanges are summarized in FIGS. 11 and 12, and the thickness changes areshown in FIG. 13.

Example 10

Further tests were conducted to determine the mass loss on the coresover time. Medium-permeability (35-50 mD) Indiana limestone cores,presoaked in 2 wt % KCl, were submerged in 30 g of 15 wt % HCl solutionfor 15 min. The reaction vessel was closed with perforated paraffin filmto allow the CO₂ to escape; the evolution of CO₂ was used to determinethe extent of reaction. Masses were recorded every 10 s for 5 min, andevery 20 s for the following 10 min. One- and two-minute intervals wereseparated and fit with a linear regression. The slopes are reported asthe minute-averaged rates as shown below.

FIG. 14 depicts the effect of CaSO₄ concentration on the overallreaction rate at different time periods as measured by CO₂ evolution,which is in general following trend of inverse proportionality.

FIG. 15 is a plot of the reaction rate over time as normalized againstthe square of remaining HCl. To normalize for the depletion of acid,these minute-averaged rates were divided by the square of the quantityof HCl remaining in solution. This metric was developed to account forthe fact that the HCl in the reaction vessel was almost fully depletedin the absence of Na₂SO₄, thus providing a more realistic correlationwith Na2SO4 concentration over a broader time window.

Examination of the reaction extent versus time revealed a dramatictrend. Na₂SO₄ addition as low as 1% by weight reduced the reaction ratesufficiently to produce a distinct reaction profile. The passivationeffect correlated positively to Na₂SO₄ concentration, as expected. At 10wt % Na₂SO₄, the core became coated with CaSO₄ extremely rapidly, andbarely reacted over the 15 min period. This configuration used stagnantacidizing fluid and did not account for the shear flow of a realfracturing treatment.

Example 11

A two-stage treatment option was explored by varying the acidconcentration when forming a CaSO₄ mask. Cores were weighed dry, thensaturated with a 2 wt % KCl clay stabilizer. The first-stage treatmentoccurred in 25 g of acid solution (5 wt %, 7.5 wt %, 10 wt %, 12.5 wt %,and 15 wt % HCl, respectively in the absence of 5% Na₂SO₄). In eachcase, the second-stage treatment was 25 g of pure 15 wt % HClirrespective of the first treatment. Exposure times of 2 min and 5 minwere also tested. Reacted cores were rinsed and dried at 225° F. (107°C.) before weighing again.

The 2-min and 5-min tests for 5 wt % Na₂SO₄ were also combined into asingle time-averaged table. While a direct comparison of mass loss ispossible, each sulfate-containing data point has its own sulfate-freereference. The proportion of mass lost in the former to the mass lost inthe latter is the factor of mass loss; this factor is reported for thevarious treatment methods tested. The results are summarized in FIG. 16.

Because of the reactor configurations, round cores were sitting flat onthe bottom of a reaction vessel. The extent to which the bottom andsides of the core reacted with the acid depended largely on the locationof the cores. The center of the bottom was largely untouched, while thesides had some noticeable etching; this is due to lifting of CaSO₄ filmby CO₂ bubbles, which caused the bottom face of the core to be exposedto the acid bath.

This test was aimed to determine the concentration of acid that providesthe largest distinction between sulfate-rich and sulfate-freetreatments. Each concentration of acid was tested for both of theseconditions, and these data points should only be compared to each other.The trend of mass loss appears to indicate a local minimum at 10 wt %HCl. This suggests that for this configuration 10% HCl provided the bestacidity for 5 wt % Na₂SO₄ to protect the surface. This result may notimply that 10 wt % HCl is the optimal concentration for anyconfiguration. ICP results were compared to a pure 15 wt % HCl treatmentat 150° F. (65.6° C.). Of particular importance is the slope of [Ca²⁺]versus time. Since the mask film required freshly generated calcium ionsto form, this system may be characterized as having a high reaction ratefollowed by a low one. The reaction rate tapered off rapidly after thetransient period of mask formation (FIG. 17).

While there was some variability in the rotating disk experiments, itproduced the most realistic conditions for an acid fracturing treatment.Elevated temperatures posed an additional challenge, as the reactionbetween HCl and CaCO₃ was accelerated. As a reference, 15 wt % HCl at150° F. (65.6° C.) and 1000 rpm would completely dissolve a core in fiveminutes.

Although the original focus of Na₂SO₄ addition had been large-scaleheterogeneity in the form of reducing the reaction rate, the potentialfor small-scale heterogeneity could be seen. The 5 wt % Na₂SO₄/15 wt %HCl reaction at 150° F. (65.6° C.) at both 500 rpm and 1000 rpmintroduced rough surface features. These features (large pits and deepvugs) would contribute to fracture conductivity upon closure.

These tests employed the same acid solution at the same temperature androtation velocity. Because fine CaSO₄ particles have the potential toplug formations and decrease the permeability of the formation, a simpletest was performed to compare the quantity of CaSO₄ formed on thesurface with the quantity formed in the bulk phase. The circular coreswere saturated with 2 wt % KCl and reacted in 25 g of acid solution of aselected concentration for 2 min. The Na₂SO₄ concentration was heldconstant at 5 wt %, and the HCl concentrations of 5 wt %, 7.5 wt %, 10wt %, 12.5 wt %, and 15 wt % were each tested. Following the reaction,spent-acid solutions were centrifuged, filtrate was separated, and CaSO₄was air-dried overnight. Cores were air-dried immediately afterreaction, weighed, thoroughly scrubbed to remove sulfate, dried, andweighed again.

While there was a visible amount of CaSO₄ in the bulk for the 5 wt % and7.5 wt % HCl cases, the mass of which was less than 0.01 g, that wasunregistered by the balance. As expected, a more acidic environment leadto less efficient mask deposition. This was likely due to the increasedagitation by CO₂ bubbles evolved by the etching reaction. Recall thatfor this configuration, 10 wt % HCl provided the best protection,measured by total mass loss. From these data, 10 wt % HCl seems to wastevery little CaSO₄ in the bulk.

Although various embodiments have been described with respect toenabling disclosures, it is to be understood that this document is notlimited to the disclosed embodiments. Variations and modifications thatwould occur to one of skill in the art upon reading the specificationare also within the scope of the disclosure, which is defined in theappended claims. Those skilled in the art will also appreciate that,although the present disclosure is presented in the context of treatingsubterranean wells, the technologies presented herein may also be usefulin applications beyond the sphere of well servicing operations.

1. A treatment fluid system, comprising (i) a first fluid comprising either: an aqueous mixture of a multivalent-cation reacting polymer; a mixture of a fatty acid and an ethanolamine; an aqueous acidic mixture of one or more compounds whose calcium salts are insoluble; or an aqueous solution comprising polyvinylpyrrolidone (PVP) and urea or alkane derivatives thereof or both; and (ii) a second fluid comprising an acid.
 2. The system of claim 1, wherein the multivalent-cation reacting polymer is sodium alginate, present at a concentration between 0.1% and 3.0% by weight.
 3. The system of claim 1, wherein the ethanolamine comprises monoethanolamine, diethanolamine or triethanolamine or combinations thereof.
 4. The system of claim 1, wherein the fatty acid has a chemical formula CH₃(CH₂)_(n)COOH, wherein n is between 10 and 24 inclusive, and the fatty acid is present at a concentration between 1.0% and 2.5% by weight.
 5. The system of claim 1, wherein the compounds whose calcium salts are insoluble comprise sodium (or other monovalent cation) sulfate, sodium (or other monovalent cation) phosphate dibasic, sulfamic acid, sodium (or other monovalent cation)-4-hydroxybenzenesulfonate, oxalic acid, tetrapotassium (or other monovalent cation) pyrophosphate, potassium (or other monovalent cation) periodate or potassium (or other monovalent cation) phosphate monobasic or combinations thereof.
 6. The system of claim 1, wherein the aqueous solution of the multivalent-cation reacting polymer further comprises urea or derivatives thereof or both.
 7. The system of claim 1, wherein the second fluid further comprises urea, derivatives thereof or both.
 8. The system of claim 1, wherein the first fluid further comprises methanesulfonic acid or alkane, benzene or naphthalene derivatives thereof or mixtures thereof, at a concentration between 0.1% and 1.0% by weight
 9. A method for treating a subterranean well having a borehole and a carbonate formation with a fracturing pressure, comprising: (i) preparing a treatment fluid comprising either: an aqueous solution of a multivalent-cation reacting polymer; a solution comprising a fatty acid and an ethanolamine; an aqueous acidic solution of one or more compounds whose calcium salts are insoluble; or an aqueous solution comprising urea or alkane derivatives thereof or both and polyvinylpyrrolidone (PVP); (ii) placing the treatment fluid in the well such that the solution contacts the carbonate formation at a pressure lower than the fracturing pressure; (iii) allowing the treatment fluid to react with the carbonate formation, thereby forming a film onto the formation surface or part of the surface; and (iv) placing an acid solution in the well such that the acid contacts the carbonate formation at a pressure lower than the fracturing pressure.
 10. The method of claim 9, wherein the multivalent-cation reacting polymer is sodium alginate, present at a concentration between 0.1% and 3.0% by weight.
 11. The method of claim 9, wherein the ethanolamine comprises monoethanolamine, diethanolamine or triethanolamine or combinations thereof.
 12. The method of claim 9, wherein the fatty acid has a chemical formula CH₃(CH₂)_(n)COOH, wherein n is between 10 and 24 inclusive, and the fatty acid is present at a concentration between 1.0% and 2.5% by weight.
 13. The method of claim 9, wherein the compounds whose calcium salts are insoluble comprise sodium (or other monovalent cation) sulfate, sodium (or other monovalent cation) phosphate dibasic, sulfamic acid, sodium (or other monovalent cation)-4-hydroxybenzenesulfonate, oxalic acid, tetrapotassium (or other monovalent cation) pyrophosphate, potassium (or other monovalent cation) periodate or potassium (or other monovalent cation) phosphate monobasic or combinations thereof,
 14. The method of claim 9, wherein the treatment fluid further comprises methanesulfonic acid or alkane, benzene or naphthalene derivatives thereof or mixtures thereof, at a concentration between 0.1% and 1.0% by weight
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